Thermionic valve and circuit



N. M. RUST ET AL THERMIONIC VALVE AND CIRCUIT Je K, i *LQE Filed Dec. 11, 1936 mu m rs 0 GRID SWING or snow ram 854M cums/ r 4/1005 (we! A/BRETT Patented June 18, 1940 UNITE STATES THERMIONIC VALVE AND CIRCUIT of Delaware Application December 11, 1936, Serial No. 115,279 In Great Britain December 16, 1935 6 Claims.

This invention relates to thermionic valves and more particularly to thermionic valves of the socalled electron beam type, i. e., of the type wherein the electron discharge par-takes more of the nature of the discharge of a cathode ray tube than of the nature of a discharge in an ordinary thermionic valve as at present in common use. Electron beam valves are described in our pending application, Ser. No. 678,128, filed June 29, 1933, issued as Patent No. 2,069,507 on February 2, 1937.

According to this invention an electron beam valve is provided, in addition to the anode and the electron gun system for projecting a beam of electrons towards the anode and for controlling the intensity ,of the said beam, with a collector electrode on the side of the anode remote from the gun, and the said anode is so slotted or otherwise shaped and is so positioned that the amount of cross section of the beam intercepted by the said anode may be varied by deflecting the beam or by varying its cross section at the anode any portion of the beam not intercepted by the anode falling on the collector electrode. The valve may include means for deflecting the beam transversely of the anode.

Important objects of the invention are: (1) To provide thermionic valves of the electron beam type exhibiting negatively sloping anode voltageanode current characteristic curves and which do not depend upon secondary emission effects, and (2) to provide thermionic valves of the electron beam type exhibiting substantially straight characteristic curves connecting grid voltage with anode current so that cross modulation and intermodulation effects may be substantially eliminated when such a valve is used in an amplifier system, c. g., in a carrier frequency amplifier for a radio receiver.

For a better understanding of the present in vention, reference is made to the accompanying drawing wherein Figs. 1, 2 and 3 represent certain characteristic curves of beam tubes according to the prior art, Fig. 4 shows a beam tube low frequency amplifier according to the present invention, Fig. 5 is an enlarged View of the anode employed in the beam tube of Fig. 4, Figs. 6, 7 and 8 are characteristic curves procured with beam tubes according to the present invention, Fig. 9 is a beam tube high frequency amplifier according to the invention, and Figs. 10 and 11 are circuit elements that may be used in place of the cathode resistors shown in the circuits of Figs. 4 and 9.

For all practical purposes electron beam valves (Cl. Nil-171) as at present known may be regarded as devices of high internal impedance whose magnification is substantially proportional to the anode load impedance.

The accompanying Figures 1, 2 and 3 show respectively anode ctu'rent (ordinates) -anode voltage (abscissae), anode current (ordinates) grid voltage (abscissae) and mutual conductance (ordinates) -grid voltage (abscissae) characteristic curves of typical electron beam valves as at present known. It may be shown, from a consideration of these curves that cross modulation effects can be produced when a strong signal and a weak signal are simultaneously applied to the control grid of a valve exhibiting such characteristics. In Figure 1 the line AB represents the dynamic load line of a weak in-tune signal and the broken line represents the load line of a strong out-oi-tune signal, the slope of this line (which would be more correctly represented by an ellipse) depending upon the amount by which the signal is out of tune with respect to the tuning of the anode circuits. The effect of the strong signal is instantaneously to shift (on the positive hall of the voltage swing) the dynamic load line of the weaker signal into a region where the mutual conductance increases and (on the negative half of the voltage swing), into a region where it decreases, with the result that the amplification of the weak signal is instantaneously increased in the former case and decreased in the latter case. This shifting of the load line is represented in Figure l by the lines parallel to AB. Known electron beam valves have a grid voltageanode current characteristic which substantially follows a square law over a considerable portion of its length and although with such a square law characteristic cross modulation eiiects will not be produced if the subsequent detection process is rectilinear, if it be not rectilinear cross modulation effects will be produced and in any event intermodulation effects which may be very serious in low frequency amplifiers are apt to be produced. It will be seen that the damaging effects are substantially proportional to the anode current changes produced by a disturbing signal and the present invention may accordingly be utilized to provide a valve adapted for use in such manner that correction may be obtained under the control of the electron beam current.

Since the typical anode voltage-anode current characteristic of an electron beam valve is substantially flat once it has risen to its maximum value, and since in carrying out the present invention the collector electrode in cfiect "robs" the anode of current by reason of the fact that the interception of the beam upon the said anode is varied, the objects of the invention may be achieved by varying the beam cross sectional area at the anode so as to modify the normal anode current anode voltage characteristic or the grid voltage-anode current characteristic as required.

The electron beam may be deflected laterally with respect'to the anode, or variation of the electron beam cross sectional area at the anode may be effected by means provided specially for this purpose or by reason of variation of the potential of one of the electrodes already mentioned.

In one form of valve (illustrated in the accompanying diagrammatic Figure 4) in accordance with this invention, there is employed a cathode l, a control electrode 2 (or control grid), an accelerator anode 3 (or screen grid), and a screening electrode 4 (or grid for minimizing secondary emission effects), all as known per se in electron beam valves and arranged in succession in the order stated close to one another with their apertures in register so that together they constitute a screened electron gun system capable of projecting an intensity controlled narrow jet or ribbon-like beam of electrons towards the anode 5. The said anode 5 is, however, not, as is at present usual, simply a plate but is provided with an elongated slot or slit 5a which is shaped and positioned in accordance with requirements to be discussed later herein. This slotted electrode is shown in enlarged face view in the accompanying Figure 5. Behind the anode, i. e., on the side thereof remote from the electron gun is a col- 5 lector electrode 6. Between the electron gun 'system and the anode is a pair of electrostatic de flector plates 1, 8, one upon each side of the normal path of the electron beam.

A valve as above described may be used as a low e the cathode input lead. Alternatively the input potentials to be amplified may be applied between the control electrode 2 and the negative terminal 9 of the source of anode potential, The accelerator electrode 3 is positively biased (for' example, to 150 volts positive), the anode 5. is biased to a still higher positive potential (for example 200 volts, positive) and the collector electrode 6 is biased to an even higher positive potential (for example 250 volts positive). The positive anode potential is applied to the anode through a resistance I2 which serves as the output resistance for the valve.

The slotted anode is so positioned that when the valve is operating over that portion of the anode current-input voltage characteristic where nocorrection eifectis required, the electron beam is wholly intercepted by the unslotted part of the anode; in other words, for what may be termed the variable mu portion of the characteristic the collector electrode 6 receives no current and the whole electron beam passes to the anode 5. At, however, that point a: on the char acteristic (Fig. '7) where the anode current-grid voltage curve begins to straighten out the edge of the electron beam just touches the edge of the slot 5a in the anode 5 so that rise in electron beam current beyond this value will cause an increased portion of the beam to fall upon the slot 5a and accordingly to pass therethrough to the collector electrode 6. The portion of the beam going through the slot 5a. of course does not constitute part of the current in the output anode circuit but is, as stated, collected by the collector electrode. Deflection of the beam in dependence upon electron beam current of course occurs by reason of the fact that the potential difference between the deflecting plates is that set up across the resistance l0 inthe cathode lead of the valve. In this way correction is obtained by deflecting the beam in dependence upon electron beam current and utilizing the collector electrode to rob the anode current by the amount necessaryto achieve the desired correction.

As will be obvious, any of a wide variety of different grid voltage-anode current curves can be obtained by suitably shaping the slot in the anode and suitably positioning the said slot, and also by suitably determining the degree of deflector control exercised. In general, however, the

arrangement will be made such as to produce a substantially rectilinear grid voltage-anode current curve, This is illustrated in the accompanying Figures 6, '7 and 8 which are graphical figures corresponding respectively to Figures 1, 2 and 3. In Figures 6, '7 and 8 the thin lines represent the ordinary electron beam valve characteristicsi. e., in these curves the current ordinates are total emission current valuesand the thick and broken line curves represent anode current characteristics obtainable by robbing" the anode in 5 accordance with this invention.

In embodiments of the invention wherein deflection of the beam is resorted to, such deflection may be made dependent either upon the current passing to the anode or upon the total electron beam current depending upon the manner in which the deflecting electrodes are connected.

It will be appreciated that some small less in efliciency is involved by reason of the robbing action but this is of minor importance compared to the advantage obtainable.

It has been found that in embodiments employing beam deflection the amount of deflection required is quite small. Substantially complete correction (resulting in a substantially rectilinear anode current-grid voltage characteristic) can be obtained with maximum deflection of the beam by an amount less than half its width, i. e., in a practical case, with a maximum deflection of less than mm, Thus the anode need not be made unduly large while, furthermore, the nece sary deflecting potentials can be obtained without having to provide an unduly large impedance from across which the said potential is derived.

Probably the practical limitation to the operation of a low frequency amplifier as illustrated in Figure 4 is set by stray capacity across the resistance l0. In practice this stray capacity which tends to by-pass the signal frequencies and reduce the voltage across resistance Ill is-in the case of a valve having an indirectly heated cathodeprincipally made up of the capacity between cathode and heater.

When the input potentials to be amplified are applied between the control electrode 2 and the end of the cathode lead resistance l0 remote from the cathode, there is obtained a degree of cathode back coupling which is also useful from the point of view of obtaining correction against undesiredcurvature of characteristic. This type of correction tends to straighten out the part of the characteristic curve which is not affected by the action of the deflecting electrodes-4. e., bottom bend curvature-but since it results in reduction of the slope of the remainder of the characteristic curve it would not ordinarily be resorted to except in cases where it is important to straighten out the bottom bend of the characteristic and sacrifice of amplification to this end is permissible.

There will now be described with reference to the accompanying Figure 9 a form of high frequency amplifier using valves in accordance with this invention and wherein it is possible to earth the cathodes and thereby avoid limitations due to stray capacity across cathode impedances.

For the sake of clarity and simplicity the amplifier will be described as and is illustrated as a two stage amplifier thoughobviously any number of stages may be provided. In this amplifier two similar valves V1, V2 each as illustrated in Figure 4: are employed in cascade, and in each case the cathode I is connected to the screening electrode 4 and to one ('I) of the deflecting plates 7, 8, the other deflecting plate (8) being connected to the cathode through a resistance It. Again as before positive potentials are applied to the electrodes 3, 5 and 6, the collector electrode potential being the highest, and the anode potential being intermediate the accelerator electrode and the collector electrode potentials. The cathodes I are earthed and connected to the neg ative terminal 9 of the anode potential source. Input potentials to be amplified are applied via a tuned circuit I3, one end of which is connected to the control electrode 2 of the first valve V1 and the other end of whch is connected to a tapping point It upon a biasing-potentiometer resistance I5 which is connected between the terminal 8 and the negative terminal I6 of a a grid bias source. The portion of this potentiometer resistance in the control grid circuit of the first valve is shunted by a condenser II. The anode of the first valve is connected through a parallel tuned circuit 58 in series with a choke I9 offering high impedance to the frequencies to be amplified to the source of anode potential and the junction point of the said choke with the said tuned circuit is connected to that end of the first valve cathode lead resistance III which is remote from the cathode, through a condenser 20 of low impedance to the frequencies to be amplified. The anode of the first valve V1 is also coupled to the control grid of the second valve through a coupling condenser .ZI, said control grid being connected to the tap I4 through a grid resistance 22. The anode of the second valve V2 is similarly connected through a parallel tuned circuit in series with a choke ofiering high impedance to the frequencies to be amplified to the positive terminal of the source of anode potential and, as in the case of the first valve, the junction point of this choke with this tuned circuit is connected to the end of the cathode resistance remote from the cathode of the second valve through a condenser of low impedance to the frequencies to be amplified. Amplified output is taken from the anode circuit of the second valve. Like parts in the two stages are indicated by like references.

It will be seen that with this arrangement both cathodes are at earth potential and good decoupling as respects the high tension supply source is obtained, for, by reason of the high impedance choke-low impedance condenser combinations, alternating currents are forced to return to the cathodes through the cathode resistances. Since the cathodes are at earth potential the cathode-heater capacities will no longer be shunted across the cathode resistances. In fact practically the only stray capacities across the said cathode resistances will be those of the low impedance decoupling condensers to earth and, by employing as decoupling condensers condensers of the insulated housing type, rather than of the metal housing type, such stray capacities can be reduced to quite small amounts.

It is not necessary to use simple resistances as the cathode resistances. For example, each simple ohmic cathode resistance may be replaced by fiatly tuned impedance network such as that shown in the accompanying Figure 10"and consisting of an inductance IOL shunted by a resistance IDR. and by a condenser IIJC and tuned at about the middle of the range of frequencies to be amplified, the dimensioning being such that the impedance over this range is substantially equal to the desired impedance value. Or again, each simple ohmic cathode resistance may be replaced by a network as shown in the 5" accompanying Figure 11 and consisting of an inductance HJL' shunted by a resistance IDR in series with a capacity IOC" shunted by a resistance IUR". With this type of arrangement if each resistance is made equal to the square root of the quotient obtained by dividing the capacity into the inductance, an over-all impedance equal to the value of each resistance will be obtained.

The accompanying Figure 12 illustrates an em- 5 bodiment of the invention where deflecting plates are dispensed with and instead variation of the cross section of the beam is obtained. Here the valve comprises an electron gun system consisting of a rectilinear cathode I coaxially situated I within a cylindrical control electrode or grid 2 which has a slot running parallel to the axis of the cathode, the electron beam emerging from this slot. After passing the slot in the control electrode the electron beam (indicated by chain -1 lines) traverses the slot in a slotted accelerator electrode 3 and then traverses the slot in a slotted screening electrode 4, the slots in the control, accelerator and screening electrodes being positioned to pass the beam. The screening, accelerator, and control electrodes together constitute a screened electron gun system adapted to project a ribbon-like beam of electrons whose intensity may be varied by applying input potentials between the control electrode and the cathode.

The anode 5 is formed as a slotted plate with a slot 5a. running parallel to and positioned in line with the slots in the other electrodes. Behind the slot 5a is a collector electrode 6 which in use is retained at a potential considerably higher than the anode potential.

With this arrangement the rays in the electron beam proceeding from the cathode will pass through a focal line or area situated somewhere between the cathode and the anode and the longitudinal position of this focal line or area may be varied in various ways-for example, by varying the anode voltage. With a low voltage upon the anode the beam will be as shown in Figure 12 with the focal line or area nearer the cathode than with a high voltage on the anode (this case is illustrated in the accompanying Figure and accordingly, for the former circumstance (Figure 12) the area of the ribbon-like electron beam 75,

at the anode will be greater than for the latter circumstance (Figure 13). In other words, one result of increase in anode voltage will be to cause the focal position to move in the direction of the anode and the beam at the anode will accordingly become narrow.

Since the anode is slotted and there is a collector anode behind its slot, the proportion of the electron beam taken up by the anode will vary with the area of the beam at the said anode, and therefore with the anode voltage. anode voltage becomes high enough the whole electron beam will pass through the slot 5a (this case is shown in Figure 13) and fall upon the collector electrode 5 and, for this operating condition, there will be no anode current.

It is accordingly possible by suitably choosing the parameters of the valve and valve circuit to modify the normal fiat anode current-anode voltage characteristic of a known electron beam valve to produce almost any desired negative slope. In this way it is possible to obtain the result so that the increased voltage upon the anode (beyond a certain value) produces a reduction of anode current, thus giving a negative resistance effect without depending upon secondary emission efiects. Similarly by suitably adjusting the parameters of the valve and its circuit a substantially rectilinear grid voltageanode current characteristic curve may be obtained.

The valve may be used as an ordinary electron beam valve by connecting the" collecting electrode and the anode together externally of the valve envelope.

It is obviously of considerably practical advantage to be able without constructionally altering a valve to control its operating characteristic and such control may be obtained with a valve in accordance with this invention either auto-- matically in dependence upon some desired varying input or output potential quantity or manually by providing manually variable operating potentials.

Typical practical dimensions for a valve as shown in Figures 12 and 13 are:

Cathode length (at right angles to the plane of the drawing) 10-25 mm.

Lengths of the slits in the various electrodes; about the same as the active cathode length. Cathode diameter: 0.5 mm.-1 mm. Width of slit in electrode 2: about 0.5 mm. Separation between electrodes 1 and 2: about Width of slit in electrode 3: about 0.5 mm. Separation between electrodes 2 and 3: 025-05 Width of slit in electrode 4: 1 mm.-1.5 mm. Width of slot 5a: 0.5 mm-l.5 mm. 7

The above figures are, of course, purely emplary.

The invention is of wide application but will be found of particular advantage when employed in pre-selector or high frequency stages in superheterodyne receivers. By reason of the freedom from cross modulation and similar distortion effects obtainable by use of this invention when the said invention is employed in a high frequency stage of a superheterodyne receiver, it enables full advantage to be taken of very high selectivity in the intermediate frequency stages for, of course, no amount of selectivity in an intermediate frequency stage will obviate interference of the cross modulation type.

Another important application of the inven- If the tion is to very high quality low frequency aniplifiers where the elimination of harmonic distortion i. e., the introduction of undesired harmonies is of importance.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed we declare that what'we claim is:

1. A circuit for the amplification of signal frequencies comprising an electron beam tube provided with a slotted anode and an electron gun for projecting an'electron beam towards said anode and for controlling the beam intensity, said electron gun being constituted by a cathode, a control electrode, an accelerating anode and a screen electrode arranged in the order named, a collector electrode positioned on that side of the slotted anode remote from' the cathode to receive electrons which do not impinge on said anode, means interposed between the screen electrode and the slotted anode for controlling the electron beam so that the amount of the cross section of the beam intercepted by said anode may be varied, progressively increasing potentials impressed respectively upon the accelerating anode, the slotted anode and the collector electrode, and a load impedance connected between only the slotted anode and the cathode.

2. A circuit according to the invention defined in claim 1 wherein the anode is provided with an elongated slot and the electron gun is adapted to project an electron beam upon said anode in a direction normal to said anode, said electron beam in cross-sectional area being elongated in the same direction in which the slot extends in the anode.

3. A circuit according to the invention defined in claim 1 whercin'the tube is provided with a pair of deflector plates interposed between the electron gun and the anode, one on each side of the electron beam path, and means for maintaining said deflector plates at different operating potentials.

4. A circuit defined in claim 1 wherein the tube is provided with a pair of deflector plates interposed between the electron gun and the anode, one on each side of the electron beam path, a connection from one of the deflector plates to the cathode for maintaining said plate at cathode potential, a resistance connected to the cathode, and a connection from the second deflector plate to the low potential end of said resistance, whereby said second plate is maintained at a negative potential with respect to the cathode.

5. A circuit arrangement according to claim 1 wherein the electron beam tube is provided with an indirectly heated cathode and an impedance connected in the cathode leg characterized in that the said cathode is earthed, and the anode circuit is completed to the cathode through said impedance and a low impedance condenser in series, the side of said condenser remote from said impedance and said cathode being connected to a source of anode potential through a high impedance choke whereby alternating currents of operating frequency are constrained to return to the cathode through said impedance.

6. A circuit for the amplification of signal frequencies comprising an electron beam tube provided with a slotted anode and an electron gun for projecting an electron beam towards said anode and for controlling the beam intensity, said electron gun being constituted by a cathode,

a control electrode, an accelerating anode and gressively increasing potentials impressed respectively upon the accelerating anode, the slotted anode and the collector electrode, and a load impedance connected between only the slotted anode and the cathode.

NOEL MEYER RUST. GEORGE FAIRBURN BRETT. 

